The Kli Yakar says that Moshe's Mussar ot Bnei Yisroel when he says in Parshas Devarim, "Bein Paran U'Bein Tofel" refers to the period between 17 Tammuz and 9 Av. Tofel refers to the Chet HaEgel (tofel is from "Tifla" folly which is refers to avodah zara) which happened on 17 Tammuz and Paran refers to the Miraglim which happened on Tisha B'Av.
When Bnei Yisroel is in trouble we can survive either because the mitzvos we perform will merit Hahsem's help or if we are united and have achdus even if we do not deserve it we will be saved. With the Chet of the Egel which was against Hashem we lost His divine protection. With the Chet of the Meraglim which caused sinas chinam we lost our unity and our second possible saving grace.
The Kli Yakar further explains that the Mazal of Tammuz is Sartan, the crab. The crab walks backwards just like we turned from Hashem with the Egel. The Mazal of Av is Aryeh since on Tisha B'Av we started to devour each other like lions. This date was not coincidentally the destruction of the Bais HaMikdash which was destroyed because of Sinas Chinam.
Moshe tells Bnei Yisroel you have put yourselves in a precarious situation by losing protection from both ends and that is why we are exposed to our enemies during these three weeks as the pasuk says in Eicha "Kol Rodfeha Hisiguha Bein HaMitzarim". These are the days that we are defenseless against our mortal enemies.
http://www.revach.net/moadim/the-3-weeks/Parshas-Devarim-Kli-Yakar-3-Weeks-Between-a-Rock-and-a-Hard-Place/592